1) Field of the Invention
The invention relates to a method in connection with a paper machine or a paper web finishing apparatus, the method comprising measuring a variable representing runnability of a paper web to be manufactured with a paper machine or processed with a finishing apparatus, and adjusting the speed difference between operational groups of the paper machine and/or the finishing apparatus on the basis of the measurement of said variable.
The invention also relates to equipment in connection with a paper machine or a paper web finishing apparatus, the equipment comprising at least one measuring means for measuring a variable representing the runnability of a paper web to be manufactured with the paper machine or processed with the finishing apparatus, and means for adjusting the speed difference between operational groups of the paper machine and/or the finishing apparatus on the basis of the measurement of said variable.
2) Description of Related Art
The production efficiency of a paper machine is directly proportional to its runnability, because disturbances in the runnability of the paper machine correlate directly with the produced paper tons and thus weaken the production efficiency. In order to pass the paper web through the paper machine without problems, the paper web should have a specific running tension throughout its manufacture, which can be achieved by adjusting speed differences between different operational groups of the paper machine by controlling runnability components of the operational groups. If the running tension is not sufficient, the following operational group of the paper machine cannot forward paper at the pace at which the previous operational group transports it. As a result, the paper web folds and breaks. Also a running tension, which is too high, causes the paper web to break. Operational groups of a paper machine include a wire section, a press section, a dryer section, which is typically divided into five or six different operational groups, a calender and a reeler. Runnability components of the operational groups include, for instance, driving rolls and suction rolls driven by an electric motor and transporting the web or felt of the paper machine, and suction rolls driven by an electric motor. In addition, the paper machine comprises freely rotating guide rolls supporting and guiding the web or the felt, and stretcher rolls, which maintain the proper tension of the wire or the felt.
Due to material-technical properties of paper it is not sufficient that the paper is provided with a specific tension only once, but the paper has to be drawn many times at many different points during the manufacture to maintain the tension. When the paper is provided with a specific tension in a certain drawing space, i.e. between the different operational groups, the web tension relaxes very rapidly when the drawing effect ceases and the web has a ‘relaxation tension’. To preserve the runnability, the relaxation tension of the paper web must be higher than the required running tension. When the tension is considered, the most critical place is between the press section and the dryer section where the speed difference between the operational groups should typically be 2 to 3% to achieve a specific tension. In the dryer section, as the paper dries, it starts to shrink and forms part of the required running tension by itself, whereby the required speed differences between the different operational groups are considerably smaller. At the end of the dryer section the speed difference can even be negative, because the web has shrunk as a result of the drying.
Drawing of the paper web with different dry solids contents during the manufacture affects the final properties of the paper. During paper manufacture, plastic, elastic and inner elongation occurs in the paper while being drawn. The relations of these components mainly depend on the dry solids content of the web. Plastic elongation takes place when the paper is wet, which means that it occurs chiefly in the press section. As the paper starts to dry, plastic elongation is not so likely to occur as the others are. In the drawing between the press section and the dryer section, paper stretches plastically so that fibers slide into each other, whereupon the fiber kinks straighten out and the web's grid-like structure becomes more regular. As a result, tensile strength of the paper increases. The optimum point can, however, be found for this wet drawing, and when the draw to the web is higher, the web RBA (Relative Bonded Area) decreases and the structure of the web becomes weaker. When the paper starts to dry in the dryer section, hydrogen bonds are formed between the fibers. When the grid-like paper web dries, the web starts to shrink and an inner tension is formed in the web, and the less the web is let to shrink, the higher the tension is. If the moisture content of the paper web is not the same in every point in the width or cross direction of the paper web, different elongation components are stored in the web due to the combined effect of the moisture content and speed differences, which can be detected, for instance, as variations in the web tension when the paper is reeled.
Paper web moisture is thus one of the variables representing the runnability of the paper web. Paper web moisture indicates how great a force is allowed for drawing the paper web between different operational groups, i.e. how big a speed difference can be arranged between different operational groups of the paper machine. DE publication 19 956 752 discloses how speed differences between operational groups of a paper machine are adjusted according to the moisture content of the web. According to the publication, the moisture content of the paper is measured by a moisture sensor arranged in the press section and/or the dryer section and arranged to measure the web moisture only at one point or location in the width direction of the web. On the basis of this point-like moisture information, the drives controlling the speeds of the operational groups of the paper machine are controlled. The point-like measurement involves a big risk, however, that as to the moisture variation in the cross direction of the web, the measurement is carried out at the point where the web moisture content is at its minimum or at its maximum or therebetween. In such a case, the speed differences can be adjusted on false grounds and not according to the real moisture content of the web, whereby compared with the tensile strength of the paper web, the web is easily drawn with too great or too low a force in the drawing spaces, which causes that the web breaks or that the web properties impair.
It is also known that paper web tension is measured in order to adjust speed differences between different operational groups of the paper machine. In a known solution, a paper web tension profile is measured, which is used for defining an average tension for the paper web, according to which the speed differences are adjusted. When the mean tension of the web is used and the tension profile is poor, i.e. highly uneven, the web tension may at some points become too high, which causes the web to break.